Catalytic oxidation of naphthalene

ABSTRACT

NAPHTHALENE IS OXIDIZED IN A LIQUID PHASE REACTION WITH AQUEOUS SULFURIC ACID AND A CATALYTIC AMOUNT OF B-FORM LEAD DIOXIDE. THE OXIDATION PRODUCTS ARE 2-NAPHTHOL AND 1,4-NAPHTHOQUINONE. THE CATALYST MAY BE FORMED ELEC-   TROLYTICALLY OR THERMALLY, AND THE OXIDATION STEP CAN BE CARRIED OUT IN AN ELECTROCHEMICAL CELL.

Dec. 4, 1973 BERN$TE|N ET AL 3,776,824

CATALYTIC OXIDATION OF NAPHTHALENE Filed July 17, 1972 United States Patent r 3,776,824 CATALYTIC OXIDATION OF NAPHTHALENE Irwin .B. Bernstein, Homewood, Ill., and Abraham Cooper, East Brunswick, N.J., assignors to The Sher- Win-Williams Company, Cleveland, Ohio 1 Filed July 17, 1972, Ser. No. 272,521

Int. Cl. C07b 3/00; C07c 49/66 12 Claims ABSTRACT OF THE DISCLOSURE Naphthalene is oxidized in a liquid phase reaction with aqueous sulfuric 'acid and a catalytic amount of fi-form lead dioxide. The oxidation products are 2-naphthol and 1,4-naphthoquinone. The catalyst may be formed electrolytically or thermally, and the oxidation step can be 'carried"out' in" an electrochemical cell.

BACKGROUND OF THE INVENTION This invention relates to oxidation processes for .& naphthalene. In particular, it relates to the catalytic oxidation of naphthalene in a multiphase process comprising an organic liquid phase-and an :aqueous sulfuric acid phase in the presence of a catalytic amount of p-form "*lead dioxide; 7

2-naphthol (beta-hydroxynaphthalene, (C H O) is presently made by a sulfonation/fusion process wherein naphthalene is" sulfonated in the 2-position by reaction with sulfuric acid. In the typical process, the sulfonic 'ac'id sodium salt is"used'with NaOI-I to convert the sulfonic acid salt to Z-naphthol. Such prior art processes are described in US. Pats. 1,922,813 and 2,760,922. FZ-haphtholhas been used for the manufacture of organic I pharmaceuticals, dyes, perfumes and for antioxidants tor -synthetic rubber.

1,4naphthoquinone (1,4-diketonaphthalene, C H O -has"-been prepared byoxidation of 1,4-aminonaphthol with chromic acid. This compound may be used as a precursor for anthraquinone by known synthetic methods.

The known methods for making these oxidation prodnets of naphthalene are relatively complex. Undesirable by-products are often difficult to separate from the desired oxidation products, and the reactions used in prior art syntheses. are usually costly to perform.

In some prior art processesfor conversion of benzenoid feedstocks to oxidation products, electrolysis has been utilized. For instance, in US; Pat. No. 2,130,151 benzene is converted tox quinone by electrolyzing a lead anode andthen passing an electrolytic current through a mixture of benzene and dilute sulfuric acid. In this process it was found that too high a concentration of sulfuric acid results in destruction of the quinone product, and

the preferred acid concentration was about 10% H 80 In US. Pat. No. 2,285,858 a similar electrolytic process for making p'-benz'oquinone from benzene by electro- BRIEF SUMMARY OF THE INVENTION It has been discovered that 2-naphthol and 1,4-naphthoquinone can be made from naphthalene by catalytic oxidation with sulfuric acid using a particular crystalline form of lead dioxide. The oxidation process may be carried out in an electrochemical cell by suspending a naphthalene-containing organic liquid phase in an aqueous "ice sulfuric acid phase containing about (25-40 vol., percent) 3.5-5 molar H SO The lead dioxide catalyst can be made by electrolysis of a lead anode to produce an electrode coated with a layer of tetragonal (/3) crystal form Pb03. The process can be carried out at ordinary temperatures. In the preferred embodiments of the invention the naphthalene precursor is dissolved in an inert organic liquid carrier such as carbon tetrachloride and the organic liquid phase is agitated vigorously during the oxidation step to provide adequate contact with the aqueous phase and the lead dioxide catalytic surface.

The mildness of the oxidation step prevents decomposition of the unconverted naphthalene feedstock, and permits recovery of the raw materials for recycle. The use of a compartmented electrolysis cell having a porous diaphragm separating the anode compartment from the cathode compartment prevents undesired reaction of the products.

THE DRAWING The single figure of the drawing is a schematic representation of an electrochemical cell in vertical crosssection.

DETAILED DESCRIPTION Referring to the drawing, there is shown a typical electrochemical cell including a vessel 10 separated into compartments by a porous wall or diaphragm 12. The cathode compartment contains a catholyte 14 in which is immersed a cathode 16. The anode compartment contains the anolyte 20 and catalytic Pb/PbO anode 22. The anolyte is agitated by a turbine stirrer 24 or like device. The electrochemical cell may be provided with a cooling system to control the temperature of the reaction, for instance, a water-cooled coil contained in the anode compartment. The electrolyte may be removed from the cell for heat exchange. It may also be removed continuously or incrementally for recovery of the oxidation products and replenishment of the organic phase and inorganic phase of the anolyte. The oxidation process may be conducted batch wise, continuously or semi-continuously according to the process or equipment requirements of the individual system.

The configuration of the lead anode may be any convenient shape, such as cylinder, plate, rod or wound helix. A perforated shape is preferred. The outer container of a concentric compartmented electrolysis cell may be constructed of lead metal and subsequently electrolyzed to form a B-form Pb0 coating on the interior surface in contact with the sulfuric acid anolyte.

Electrochemical formation of ,B-form tetragonal lead dioxide can be obtained in a conventional process cell. The desired electrode shape is made using pure lead metal stock, such as perforated sheet. The Pb/PbO' electrode is then formed in situ in situ by pre-electrolyzing the lead electrode at low current density, typically about 0.15 amp./m. to form the lead dioxide film. Electrolytic methods for making the tetragonal lead dioxide used as catalyst herein are described by Cooper and Mantel in I&EC Process Design and Development vol. 5, 238 (July 1966) and byRuetschi and Angstadt in J. Electrochem. Soc. 111, 1329 (1934). The electrochemical formation of S-form PbO is performed at an anodic potential of at least about 2.5 volts with reference to a standard calomel electrode connected to the lead anode in the conventional manner. The anode potential during formation of the catalyst and during electrochemical oxidation can be measured with a standard laboratory vacuum tube voltmeter. For use in measuring anode potential, a Beckman No. 34079 calomel electrode is held in direct contact with the anode and connected to a Hewlett-Packard Model 413A DC nul vacuum tube voltmeter.

It is preferred that the electrochemical cell be compartmented to prevent undesired reaction of the oxidation products. The anolyte and catholyte may be separated by a porous diaphragm. Such separators can be constructed of woven inert organic fibers, such as acrylic, olefin or halogenated olefin polymers, or inorganlc materials such as asbestos may be used. High quality diaphragms may be constructed of Alundum or semi-permeable membranes. Cation or anion per selective membranes, well known in the electrochemical arts, may be used to prevent migration of undesired ions or molecules across the cell separator between the electrolyte bodies.

The construction of the electrolytic cell may be of standard materials. Glass-lined metal, inert plastics and rubber may be used for the vessel.

The choice of materials for the cathode and the ionic components of the catholyte are largely matters of choice for the skilled electrochemist. The cathode may be made of electrically conductive material, including steel, carbon, lead, etc. The catholyte can comprise a non-deleterious solution of inorganic acids, bases or salts. For purposes of simplicity, the catholyte may be of the same composition as the anolyte aqueous phase, e.g., 4.5 M H 80 The catholyte may contain metal ions, such as Na+, and basic anions, such as OH.

Electrical connections can be made with PTFE-insulated copper conductor. The electrodes are connected to a suitable source of direct current. A solid state power supply with regulated output and voltage regulation may be used. For laboratory scale electrolysis a Lambda Model 13-104, rated at 1 kva. and having a regulated voltage of -36 volts, is suitable.

The anolyte for electrochemical oxidation of naphthalene comprises an organic phase suspended or dispersed in an aqueous acid phase. Initially the organic phase consists essentially of the naphthalene feed stock and an inert organic carrier or solvent for the naphthalene. During the reaction, oxidation products may be permitted to accumulate in anolyte, particularly in the organic phase. These products may be removed continuously or incrementally and the anolyte components may be replenished as needed.

The inert organic solvent should not be deleterious to the oxidation reaction. This component should be liquid under reaction conditions and preferably is a non-viscous halogenated paraffin hydrocarbon, such as carbon tetrachloride. Other liquid organic carriers or solvents may include bromotrichloromethane, carbon tetrabromide, heptachloropropane, tetrachloroethylene, or mixtures of these with one another. Typically, the organic phase contains about 5 to 50 parts by weight of naphthalene per 100 parts of liquid organic carrier or solvent. The organic solvent can be separated from the other organic anolyte components by extraction or evaporation methods.

The aqueous acid phase of the anolyte should contain sulfuric acid equivalent to about 3.5 to 5.0 M sulfate ion. The preferred aqueous anolyte consists essentially of about 25 to 40 vol. percent H 80 however, other ions may be present in relatively small amount without interfering with the catalytic oxidation reaction.

The relative amounts of organic and aqueous phases in the anolyte is best determined by the particular equipment and process variables being utilized. Ordinarily a volume ratio of about to 50 parts by volume organic phase per 100 volumes of aqueous phase is satisfactory. For purposes of economy, at least 1 part organic phase per 100 parts aqueous phase should be used. The practical upper limit of the organic phase is determined by agitation, viscosity, concentration reaction time, etc.; however, not more than about 100 parts organic phase is practicable under typical process conditions.

The reaction may be carried out in the temperature range from freezing to boiling range, usually 0 to 100 C., depending on the boiling point of the solvent. A cooling coil or like apparatus can be used to control the electrolyte temperature, preferably in the range of about 20 to 60 C. Average reaction time or residence time is usually about 30 to 240 minutes. Lower current densities tend to require longer residence times; however, there is some evidence that low current density will give better conversion of feedstock and higher yields for the oxidation product.

During the electrolytic oxidation, it is preferred that the process be conducted under constant current conditions. Anodic current density of less than about 25 amperes per square meter (amp./dm. is satisfactory, with the preferred anodic current being in the range of 6.25 to 25 amp./dm. in the operating temperature range near ambient. Careful control of the anodic current density can give better conversion of naphthalene and better yield of the desired oxidation products.

The cell voltage depends upon electrolyte concentration, anode and cathode area, electrode spacing, diaphragm type, operating temperature, etc. For a typical electrochemical oxidation process the anode voltage is about 2.5 to 5 volts DC for a batch process having a residence time of about 1 hour.

The anolyte can be removed from the reaction vessel for treatment. The organic and aqueous phases of anolyte are easily separated by settling and centrifuging, or in a separatory funnel. The sulfuric acid may be discarded or washed with solvent and replenished for use again. The unconverted naphthalene and organic solvent can be recovered from the oxidation products for recycle.

While a precise explanation of the reaction theory for this reaction may not be possible, a brief consideration of a proposed mechanism may facilitate understanding of the invention. The discussion of theory is intended only to advance comprehension of the reaction and not considered binding upon the claims appended to this specification.

Most likely, the reaction proceeds by attack on one or more tertiary hydrogen atoms on the naphthalene nucleus by an oxygen atom of lead dioxide with formation of the free radical.

H-OzPbzO- The free radical then loses an additional electron to a second oxygen atom to form the carbonium atom, which reacts with water to form the protonated naphthol. This in turn loses a proton to form the corresponding naphthol. This portion of the proposed mechanism is shown below.

effectiveness of this catalyst 5 grams of naphthalene is dissolved in about 40 grams of carbon tetrachloride inert solvent and agitated sulfuric acid (25 vol.) and 2 grams lead dioxide powder catalyst. The catalyst is separated from the reaction mixture by filtering. The catalyst is washed with carbon tetrachloride and the washing is combined with the filtrate. The aqueous sulfuric acid phase is separated from the liquid organic phase and the inert solvent is removed from the sample by evaporation at low temperature under vacuum. The residue was analyzed for the precursor and oxidation products. About 88-90% of the naphthalene was converted. The product consists essentially of a mixture of Z-naphthol and 1,4-naphthoquinone, with more than 80% selectivity to the naphthoquinone.

While the invention has been demonstrated by particular examples, there is no intent to limit the inventive concept except as set forth in the following claims.

We claim:

1. A process for electrochemical oxidation of naphthalene which comprises:

maintaining lead dioxide catalyst in El-phase by imposing on the lead dioxide an anodically biased direct current potential greater than about 2.5 volts relative to a standard calomel electrode while contacting the lead dioxide with an aqueous sulfuric acid electrolyte, said electrolyte containing about 3.5-5 molar H 80 and being in contact with a cathode;

contacting naphthalene in an inert organic carrier with the electrically biased lead dioxide catalyst and sulfuric acid at a temperature of about 0 to 100 C. for sufficient time to convert at least a portion of the naphthalene to 2-naphthol or 1,4-naphthoquinone.

2. The process of claim 1 wherein the lead dioxide and sulfuric acid electrolyte is contained in an anolyte compartment separated from a catholyte compartment by a porous diaphragm.

3. The process of claim 2 wherein the porous diaphragm is Alundum or a permselective membrane.

4. The process of claim 1 wherein an anodic current density of about 6.25 to 25 amperes per square meter is maintained for about 30 to 240 minutes, using a direct current anodic potential of at least about 2.5 volts relative to a standard calomel electrode.

5. The process of claim 4 wherein the lead dioxide electrode is anodically pre-electrolyzed in contact with the sulfuric acid electrolyte at a current density of about 0.15 amp/dmfi.

6. A process for making Z-naphthol according to claim 1 wherein the naphthalene is dissolved in an inert liquid perhalogenated hydrocarbon.

7. The process of claim 1 wherein at least a portion of the naphthalene is converted to 2-naphthol.

8. The process of claim 1 wherein at least a portion of the naphthalene is converted to 1,4-naphthoquinone.

9. A catalytic process for oxidizing naphthalene which consists of suspending a naphthalene-containing organic liquid phase in an aqueous sulfuric acid phase containing about 3.5-5 molar H and oxidizing the suspended naphthalene in contact with a catalytic amount of a lead dioxide catalyst consisting essentially of tetragonal fl-crystal form PbOg at a temperature of about 20 to 60 C. 10. A process for making 2-naphthol which comprises contacting naphthalene at about 0 to C. in an inert liquid halogenated solvent with aqueous 3.5-5 molar sulfuric acid and a catalytic amount of tetragonal lead dioxide.

11. A process for electrochemical oxidation of naphthalene which comprises:

maintaining p-form lead dioxide on a lead anode by biasing the anode with a direct current potential of about 2.5 to 5 volts relative to a standard calomel electrode in an aqueous electrolyte consisting essentially of 3.5- 5 molar sulfuric acid.

contacting naphthalene in an inert organic carrier which the electrically biased lead dioxide at about 0 to 100 C. for sufficient time to convert at least a portion of the naphthalene to 2-naphthol.

12. A process for making 1,4-naphthoquinone which comprises heating naphthalene in an inert solvent with an aqueous solution of sulfuric acid consisting essentially of about 3.5 to 5 molar H 80 and a catalytic amount of ,B-form PbO at a temperature of at least about 40 C. to oxidize at least a portion of the naphthalene to 1,4- naphthoquinone.

References Cited UNITED STATES PATENTS 3,513,178 5/1970 Joo et al 260-396 R 3,681,401 8/1972 Joo et al 260-396 R 2,285,858 6/ 1942 Horrobin et al 204-7 8 2,130,151 9/1938 Phlfreeman et al. 204-78 3,634,216 1/ 1972 Gibson et al. 204-83 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 260-396 R, 621 G 

